If we examine closely the mathematical model of the induction motor shown in Figure 20 of the previous articles, you will notice that the flux and torque loops are not separated.
In the AC motor terminals have three wires carrying the vector sum of las.corrientes that produce flow and torque. This is the reason why the vector control AC motor is more difficult than that for a DC motor.
The challenge for control of variable speed AC motor is to distinguish both streams without the benefit of separate circuits.
Figure 3-10 shows the problem. The stator current I1 is the vector sum of the currents that produce flux and torque. The angle between IM and I2 is constantly changing under different conditions. The stator current must then be varied to produce the required torque current, while the magnetizing current must be maintained.
Since many variables are involved in the calculation of currents, then there are many ways to design the control of the drive. These include direct and indirect schemes. Direct schemes electrically measure the angle of the rotor flux. Indirect control uses field-oriented speed or position feedback of the motor and slip considerations to provide instantaneous torque command and flux.
Power PWM circuit is commonly used on three basic types of regulators. Are these regulators that determine the capabilities of the drive, including the response, speed regulation due to transient changes in load and torque capacity at low speed.
- V/F.- REGULATOR
The most common configuration, lower cost, is used in applications with or without speed feedback. This design usually offers the basic settings of a drive, including adjustment of speed, torque limit, V / Hz, voltage step at low speeds, minimum and maximum speed, acceleration and deceleration rates and other similar adjustments to meet the requirements for most applications.
The V / f control in its simplest form takes a speed reference command from an external source and varies the voltage and frequency applied to the motor.
Because maintaining a constant ratio of V / f, the inverter can control the speed of the motor connected. It is able to regulate the torque. Figure 3-11 shows the block diagram of the regulator V / f.
Typically, a current limiter block monitors the motor current command and alters the frequency when the engine exceeds the predetermined current value. The drive only works with the total motor current and can not distinguish the capacity limits of IM I2. The maximum peak torque is 150%.
The block "slip compensation" alters the frequency reference when the load changes to maintain the current speed of the engine near the desired speed.
While this type of control is sufficient for many applications, not so much when you have applications that require very fast dynamic response, such as when the engine has to work at very low speeds or applications that require direct control of the motor torque rather Motor frequency.
- BASIC VECTOR CONTROLLER
Introduced in the mid-80s, this control was a significant advance over the design V / Hz. Each unit uses an approximation method to control the angle of the rotor-stator flow to optimize engine operation. Some drives vector had the expectation of speed regulation, open loop equivalent to a DC speed controller with feedback. Many units did not come close to these expectations. Despite this, the Vector Core provides better performance.
- Sensorless vector REGULATOR
More recently, in the mid-90s, were introduced many regulators Vector improved. These were the recent advances in microprocessors and DSPs that enriched significantly the operations of the drive, including the ability to control and position response. One reason that the capabilities of the Vector work much better is the ability to "see" the EMF (electromotive force) produced by the engine, then the circuitry adjusts the start of each PWM pulse train duration and pulse-specific .
Vector drives are used in printing presses High speed presses, winders and other machinery systems coordinated. Vector drives are also used in servo positioning systems, such as rewinding machines. Some can accelerate from rest to maximum speed in a time of 1 to 200 ms.
With all these types of drives, a feedback signal speed or position drive improvement work. Figure 3-12 shows the type of controller shown, where it was decided to take the engine speed signal through a tachometer dynamo-estimate rather than an observer. In contrast, the torque generated and the magnetizing current obtained from the corresponding observer.
